The cooling of astrophysical media by HD

The results of recent quantum mechanical calculations of cross-sections for rotational transitions within the vibrational ground state of HD are used to evaluate the rate of radiative energy loss from gas containing HD, in addition to H, He and H₂. The cooling function for HD (i.e. the rate of cooling per HD molecule) is evaluated in steady state on a grid of values of the relevant parameters of the gas, namely the gas density and temperature, the atomic to molecular hydrogen abundance ratio and the ortho:para-H₂ density ratio. The corresponding cooling function for H₂, previously computed by Le Bourlot et al., is slightly revised to take account of transitions induced by collisions with ground-state ortho-H₂ ( J =1). The cooling functions and the data required for their calculation are available from http://ccp7.dur.ac.uk/. We then make a study of the rate of cooling of the primordial gas through collisions with H₂ and HD molecules. In this case, radiative transitions induced by the cosmic background radiation field and, in the case of H₂, collisional transitions induced by H⁺ ions should additionally be included.